1. Field of the Invention.
The present invention relates to a method and apparatus for redistributing radiant energy from an image formed in a spectrophotometer to obtain more accurate spectroscopic measurements of selected areas of a sample.
2. Description of Related Art.
Known beam splitters may be divided into three classes. The first class is an amplitude beam splitter which typically uses a partially reflecting/partially transmitting element to split the amplitude of an incident wave front. A common example of an amplitude beam splitter used at optical wavelengths is a partially silvered mirror. The beam splitter reflects a first portion of the radiant energy and transmits a second portion. Typically a portion of either the transmitted or reflected energy is utilized in an optical system and returned to the beam splitter where the beam splitter transmits a third portion and reflects a fourth portion of the radiant energy. It is known to maximize the efficiency of an amplitude beam splitter by transmitting and reflecting equal amounts of the radiant energy that is incident to the beam splitter. Thus, the maximum efficiency at which radiant energy may be transmitted is one half of one half of the incident radiant energy, or 25 percent. Further, the transmissive and reflective properties of an amplitude beam splitter should not change with the wavelength of the incident radiant energy if the beam splitter is used in spectroscopic applications.
In many spectroscopic applications, however, an ideal amplitude beam splitter does not exist. For example, a common amplitude beam splitter used in infrared spectroscopy is a film of germanium on a potassium bromide substrate. This type of amplitude beam splitter is expensive and not very durable. Further, a film of germanium does not produce a perfect division of energy, and its reflective/transmissive properties change greatly with wavelength at infrared wavelengths.
A second class of beam splitter is an aperture beam splitter. An aperture beam splitter involves inserting at least one intercepting mirror into a beam of radiant energy at some point along its optical path. An aperture beam splitter is reliable and easily fabricated so as to exhibit perfect reflectivity over a large range of potential wavelengths because many reflecting surfaces are known that exhibit near total reflection at wavelengths of interest. An aperture beam splitter, however, reduces the effective aperture of the optical system because the intercepting mirror reflects only a portion of the beam of radiant energy. Any reduction in aperture is undesirable for microspectrometry because a larger aperture can resolve a smaller object.
A third class of beam splitter uses a polarizer such as a wire grid or Brewster's plate. Polarizers are known that exhibit near perfect 50 percent transmission or reflection provide for full utilization of existing aperture and are reliable. A disadvantage to using a polarizing beam splitter, however, is that it illuminates the sample with only radiant energy having a particular polarization. Restricting observation to one particular polarization is undesirable for some applications because the spectrum of a sample may change at different polarizations. Further, much reflected radiant energy may be lost if the polarization is mismatched to a sample that is bifringent; e.g. if the incident radiant energy has predominantly one type of polarization and the sample reflects radiant energy with predominately the opposite polarization.
It is known that the response of detectors for commercial infrared spectrophotometers changes depending on how much of the total surface area of the detector is illuminated. For example, mercury cadmium telluride detectors, typically used in commercial FT-IR spectrophotometers, give rise to "beat patterns" when partially illuminated due to phenomena that are completely unrelated to the spectrum of the sample such as multiple internal reflectance within the detector. Obtaining a spectrum with a spectrophotometer typically involves taking a "baseline" measurement of the source by fully illuminating the detector. The spectrophotometer does not compensate for distortions produced by the detector if the detector is illuminated differently during the baseline measurement than during the spectroscopic measurement of the sample.
Irregular illumination of the detector is produced in several ways. For example, directly imaging the sample onto the detector may produce uneven illumination because of the image properties of the sample. Even more drastic changes in the illumination of the detector results when the sample is masked to limit the geometric size of the illuminated area on the sample. Often the shape of the mask combines with the image of sample at the detector to produce uneven illumination. Moreover, the foregoing irregularities in the illumination of the detector cannot be produced independent of taking the spectral measurement and therefore may not be compensated for in the baseline measurement.
Any image information contained in an image may be destroyed by forming the image at one end of a light pipe. Radiant energy diverges from the image focus and reflects off the walls of the light pipe at different locations depending on the divergence of each segment of the radiant energy beam, thus spreading the radiant energy from different portions of the image throughout the volume of the light pipe so that the image information originally contained in the image is destroyed. The radiant energy emerges from the light pipe as if from a focus that contains only an image of a homogeneous distribution of radiant energy.
It is further known in the art to employ a light pipe before the detector of a spectrophotometer to scramble an image and evenly spread the radiant energy across the entire surface of the detector and that destruction of the image information contained in the sample does not result in loss of spectroscopic information contained in the radiant energy.